The subject matter disclosed herein relates to a magnetic resonance imaging (MRI) apparatus and a magnetic resonance image displaying method. The present invention relates particularly to a magnetic resonance imaging apparatus and a magnetic resonance image displaying method each of which generates a magnetic resonance (MR) image with respect to an imaging area of a subject, based on magnetic resonance (MR) signals acquired by executing a diffusion weighted imaging (DWI) pulse sequence on the imaging area of the subject within a static magnetic field space and thereafter displays the generated magnetic resonance image on a display screen.
A magnetic resonance imaging apparatus has frequently been used for medical applications in particular as an apparatus for imaging a subject using a nuclear magnetic resonance (NMR) phenomenon.
In a magnetic resonance imaging apparatus, an imaging area of a subject is held or accommodated within a space formed with a static magnetic field thereby to arrange spins of proton lying in the imaging area in the direction of the static magnetic field, and a magnetization vector thereof is produced. An RF pulse having a resonant frequency is applied to generate a nuclear magnetic resonance phenomenon, thereby flipping the spins of the proton and changing the magnetization vector of the proton. Thereafter, the magnetic resonance imaging apparatus receives magnetic resonance signals produced when the proton is returned to the original state of magnetization vector, and reconstructs a magnetic resonance image such as a slice image with respect to the imaging area, based on the received magnetic resonance signals.
In this type of magnetic resonance imaging apparatus, it has been practiced to execute a diffusion weighted imaging pulse sequence on the imaging area of the subject within the static magnetic field space thereby to acquire the magnetic resonance signals and thereafter generate a diffusion weighted image or the like as a magnetic resonance image with respect to the imaging area, based on the acquired magnetic resonance signals (refer to, for example, Shigeki Aoki et al., “Better Understand What Diffusion MRI is”, SHUJUNSHA Co., Ltd. and Japanese Unexamined Patent Publication No. 2006-262928).
The diffusion weighted imaging pulse sequence is a pulse sequence for applying so-called MPG (Motion Probing Gradient) pulses before and after a refocusing pulse in a spin echo (SE) method for transmitting RF pulses of, for example, an excitation pulse whose flip angle is 90° and a refocusing pulse whose flip angle is 180°. A b-factor of each MPG pulse is adjusted and executed. Here, the diffusion weighted imaging pulse sequence is executed by, for example, a pulse sequence of a spin echo method to which an SE-EPI (Echo Planar Imaging) method or an FSE (Fast Spin Echo) method is applied. This pulse sequence is designated as, for example, a Stejskal-Tanner method or a PGSE (Pulsed Gradient Spin Echo) method.
Described specifically, upon carrying out the diffusion weighted imaging pulse sequence, the pulse sequence is executed by the PGSE method under conditions of plural b-factors. Thus, a plurality of diffusion weighted images arc generated with respect to an imaging area thereof so as to correspond to the conditions of the plural b-factors respectively. Here, the pulse sequence is executed on the condition that the b-factor is set to 0, thereby to generate a T2 weighted image called a b0 image with respect to the imaging area.
Using the generated T2 weighted image and diffusion weighted images, diffusion parameters each indicative of such a characteristic that water molecules contained in the imaging area arc diffused, as in the case of an eigen or intrinsic value and vector of a diffusion tensor, and a fractional anisotropy (FA: Fractional Anisotropy) value, a mean diffusivity (MD) value, a relative anisotropy value, etc., are calculated to generate a diffusion tensor image. A magnetic resonance image such as a fractional anisotropy image in which fractional anisotropy values are mapped, a nerve fiber image in which each nerve fiber is imaged by tractgraphy, or the like is generated and displayed (refer to, for example, Japanese Unexamined Patent Publication No. 2004-81657).
The magnetic resonance image generated in the above-described manner is displayed on a display screen by appropriately setting a window level (WL) and a window width (WW) respectively.
When, for example, diagnostic imaging of each acute phase cerebral infarction is conducted, a window level and a window width are set to accurately identify an infarction area at a subject therefor and a diffusion weighted image is displayed on a display screen of a display so as to correspond to the set window level and window width.
Described specifically, an operator observes a T2 weighted image generated with respect to an imaging area in a manner similar to the diffusion weighted image and sets a region of interest (ROI) to an area corresponding to thalamus of a subject at the T2 weighted image. Thereafter, display conditions are set in such a manner that each pixel value in the region of interest assumes a window width and a value equal to half of the pixel value assumes a window level. That is, the window width is set in such a manner that the pixel value in the region of interest is brought to an upper limit value of brightness of a display image and a value set as the reference when each pixel value in the region of interest is determined, is brought to a lower limit value of the brightness of the display image. Further, the center value of the window width is set so as to assume a window level. Then, the diffusion weighted image is displayed on the display screen so as to correspond to the display conditions. Thus, since the diffusion weighted image is displayed on the display screen with a high contrast between an area intended for diagnosis thereat and an area other than the area, an appropriate and efficient diagnosis can be realized (refer to, for example, “Standardization of DWI (Diffusion Weighted Image) Display”, [online], [Searched on Jul. 24, 2007], Internet <URL: http//asist.umin.jp/assignment-detail01.htm>).
Since, however, the area corresponding to the thalamus at the T2 weighted image is not displayed with the high contrast with respect to other area in the above, the operator needs to set the region of interest to the area corresponding to the thalamus. It is therefore difficult to automatically set the display conditions such as the window level and the window width. Since a so-called human error might occur when the operator sets the region of interest to the area corresponding to the thalamus at the T2 weighted image, there is a case in which it is difficult to display the diffusion weighted image on the display screen at a proper window level and width. With these views, it might be difficult to enhance diagnostic efficiency.
When the nerve fiber image generated by tractgraphy is displayed in addition to the above, a window level and a window width might not be set property even to each of images generated by execution of the diffusion weighted imaging pulse sequence as in the case of the T2 weighted image and the diffusion weighted image or the like each used as a background image. Therefore, there is a case in which the image is not properly displayed on the display screen and an improvement in diagnostic efficiency is difficult.